Ammonia is a waste product of cellular metabolism that is constantly generated throughout the body. Ammonia exists as 99% NH4+ and 1% NH3 at physiological pH, and is thus able to cross the blood-brain barrier by diffusing in its gas phase (NH3). Cooper et al., “The Metabolic Fate of 13N-Labeled Ammonia in Rat Brain,” J. Biol. Chem. 254:4982-4992 (1979). Ammonia homeostasis is vitally important in the brain, as an equimolar amount of ammonia is simultaneously released with glutamate during neuronal firing. Marcaggi et al., “Neuron-Glial Trafficking of NH4+ and K+: Separate Routes of Uptake Into Glial Cells of Bee Retina,” Eur. J. Neurosci. 19(4):966-976 (2004). If allowed to accumulate, ammonia causes brain dysfunction ranging from mild cognitive impairment, to seizures, coma, and death. Cagnon et al., “Hyperammonemia-Induced Toxicity for the Developing Central Nervous System,” Brain Res. Rev. 56:183-197 (2007); Butterworth, R. F., “Pathophysiology of Hepatic Encephalopathy: A New Look at Ammonia,” Metab. Brain Dis. 17:221-227 (2002). Ammonia is a ubiquitous waste product that accumulates in numerous metabolic disorders, causing neurological dysfunction ranging from learning impairment, to tremor, ataxia, seizures and coma. The brain is particularly vulnerable to ammonia as it readily crosses the blood-brain barrier and rapidly saturates its only removal pathway, glutamine synthetase, located in astrocytes. Nordström et al., “Effects of Phenobarbital in Cerebral Ischemia. Part I: Cerebral Energy Metabolism During Pronounced Incomplete Ischemia,” Stroke 9(4):327-335 (1978).
Ammonia homeostasis is particularly important in the brain, as an equimolar amount of ammonia is simultaneously released with glutamate during neuronal firing. Marcaggi et al., “Neuron-Glial Trafficking of NH4+ and K+: Separate Routes of Uptake Into Glial Cells of Bee Retina,” Eur. J. Neurosci. 19(4):966-976 (2004). Brain ammonia is almost exclusively detoxified by condensation with glutamate to form glutamine, a reaction catalyzed by glutamine synthetase with a rapid half-life of less than 3 seconds. Cooper, A. J., “13N as a Tracer for Studying Glutamate Metabolism,” Neurochemistry International 59:456-464 (2011). Interestingly, glutamine synthetase has a higher affinity for ammonia than glutamate, suggesting that the removal of ammonia is prioritized over an excitotoxic neurotransmitter. Waniewski, R. A., “Physiological Levels of Ammonia Regulate Glutamine Synthesis From Extracellular Glutamate in Astrocyte Cultures,” J. Neurochem. 58:167-174 (1992).
The current literature points to astrocytes as the primary target of ammonia toxicity, as they are the only cell type in the brain that express glutamine synthetase. Martinez-Hernandez et al., “Glutamine Synthetase: Glial Localization in Brain,” Science 195(4284):1356-1358 (1977).
Astrocytes have been shown to swell when exposed to ammonia in histological and cell culture studies. Butterworth, R. F., “Pathophysiology of Hepatic Encephalopathy: A New Look at Ammonia,” Metab. Brain Dis. 17:221-227 (2002); Jayakumar et al., “Na—K—Cl Cotransporter-1 in the Mechanism of Ammonia-Induced Astrocyte Swelling,” J. Biol. Chem. 283:33874-33882 (2008). However, there has been little progress in developing successful therapies, as the causal mechanism of ammonia toxicity remains unclear. Does astrocyte swelling contribute to the initial deterioration of neurological functions characteristic of acute ammonia toxicity, or is there a failure of other astrocytic homeostatic functions?
The present invention is directed to overcoming these and other deficiencies in the literature by targeting a novel disease mechanism that may be more important to symptom development in ammonia neuro-toxicity.